1. Field of the Invention
This invention relates to dc-SQUID magnetometry and superconducting electronics and, in particular, to a magnetometer including a superconducting SQUID having an inherent phase shift without application of external magnetic fields.
2. Discussion of Related Art
Very precise measurements of small magnetic fields can be accomplished with a dc-SQUID magnetometer device. A conventional dc-SQUID magnetometer includes a superconducting loop containing a plurality of Josephson junctions, coupled to terminals. Any change in the magnetic field which penetrates the superconducting loop disturbs the current through the device, which is detectable at the terminals. Thus, the dc-SQUID can be used as a device for measuring changes in a magnetic field.
Conventional dc-SQUID magnetometers lack inherent sensitivity. Furthermore, a conventional dc-SQUID magnetometer can only determine the magnitude of the change in a magnetic field, but cannot distinguish the direction of the change. In order to hyper-sensitize a standard dc-SQUID, flux-biasing can be used to shift the latent flux position in the SQUID loop into a linear response regime. A standard dc-SQUID loop behaves in accordance with a well defined current-phase relationship. The equilibrium position of the current-phase relation of a standard dc-SQUID lies in a region of sensitivity where the induced superconducting current is proportional to a small perturbation in the flux squared (Ixe2x88x9d"PHgr"2), and resultingly there is no directional sensitivity. By biasing the SQUID loop with an applied flux, the equilibrium position can be shifted into a more sensitive linear response regime, thus introducing directional sensitivity into the current response as well. This can be accomplished by introducing a phase shift of the equilibrium position in the current-phase relation. The phase shift is realized in conventional dc-SQUID devices by application of an external magnetic field to the dc-SQUID device, a technique called flux biasing. In other words, with an externally applied magnetic flux on the SQUID device, a small perturbation in the flux induced by the magnetic field that is being measured will result in a linear response in the superconducting current from the SQUID device.
Furthermore, by coupling multiple SQUID loops, it is possible to enhance the sensitivity of the dc-SQUID magnetometer. See U.S. Pat. No. 5,767,043, entitled xe2x80x9cMultiple Squid Direct Signal Injection Device Formed On a Single Layer Substrate,xe2x80x9d to Cantor et al., herein incorporated by reference in its entirety. One application of dc-SQUID magnetometry is as a non-destructive testing device in the field of semi-conductor electronics. In the electronics industry, each circuit that is manufactured must be non-destructively tested for correct operating parameters. This is accomplished by running current through the circuit to be tested and measuring the resulting magnetic fields. However, in order to detect flaws in the magnetic field a high degree of resolution is required, which cannot be achieved without flux biasing or coupling the dc-SQUIDs that make up the magnetometer. Thus, there is a necessity for increasing the latent sensitivity of the SQUID magnetometer.
Further applications for dc-SQUID magnetometers range in practical uses. For example, dc-SQUID magnetometers are used in Magnetic Resonance Imaging, microscopic metal defect detection, mine detection, and submarine detection. Additional examples of uses for dc-SQUID magnetometers include analogue-to-digital converters and optical switches. Given the broad range of applications of dc-SQUID Magnetometers, there is a need for devices with increased sensitivity, including directional sensitivity, wherein the overall size and cost of a device is reduced.
There are, however, practical limitations to current methods of dc-SQUID sensitizing. Biasing the loop introduces magnetic fields that may interfere with the fields or system being measured. Similarly, coupling dc-SQUIDs can lead to bulky measurement tools that increase the obtainable distance from the sample, thereby also decreasing the ability to measure magnetic fields in the sample.
The use of a phase shifter in order to sensitize the current-phase behavior in a superconducting loop is known; however, the inherent sensitization has been restricted to a xcfx80-phase shift. Thus there is a need for a device that can be used in dc-SQUID magnetometry with a high level of latent sensitivity, as well as directional sensitivity without the application of external magnetic fields.
In accordance with the present invention, a dc-SQUID magnetometer is presented which provides an inherent phase shift in a superconducting loop, i.e. a phase shift in the absence of an external magnetic field. Some embodiments of a dc-SQUID magnetometer according to the present invention include a high sensitivity, directional, superconducting Josephson device formed of a superconductive loop having a xcfx80/2-Josephson junction and a 0-Josephson junction. The superconductive loop is further coupled to at least two terminals by which a current may flow through the loop.
The superconducting materials forming the superconducting loop and terminals can have dominant order pairing symmetry with non-zero angular momentum. In some embodiments, the superconducting material can be a high temperature, d-wave superconductor such as YBa2Cu3O7xe2x88x92x, where x has values less than 0.4 and greater than 0.05, or Bi2Sr2Canxe2x88x921CunO2n+4. In some other embodiments, a dc-SQUID magnetometer according to the present invention can include a p-wave superconducting material forming 0-junctions and xcfx80/2-junctions. An example of a p-wave superconducting material includes Sr2RuO4.
Junctions having a xcfx80/2 phase shift or a 0 phase shift, for example, can be fabricated at the grain boundary of two d-wave superconducting materials. For example, in a junction formed at the grain boundary between two d-wave superconducting materials with a 45xc2x0 misalignment in their crystal lattice structures, a xcfx80/2 phase shift results in a junction that is perpendicular to the terminals of the junction. Similarly, a 0xc2x0 phase shift can be achieved in a grain boundary Josephson junction in which the misalignment in the crystal orientation between the superconductors on either side of the grain boundary is zero (in the trivial case) or, the 0xc2x0 phase shift can be achieved in the case of a symmetric 22.5xc2x0 grain boundary junction, where the a-axis of the order parameter of the two superconductors are rotated xc2x122.5xc2x0 from parallel to the junction interface, respectively.
The combination of a 0-junction and a xcfx80/2 junction induces an overall xcfx80/2-phase shift in the current as the superconducting loop is traversed, thus shifting the equilibrium position of the current-phase relation Resultingly, a xcfx80/2 dc-SQUID loop according to the present invention has a linear current-phase response with small changes in externally applied magnetic flux. The measured current is also sensitive to the direction of the flux through the loop. Further, the xcfx80/2 dc-SQUID loop does not require any externally applied flux biasing. This inherent phase shift allows for an order of 100 fold increase over the sensitivity of standard embodiments of dc-SQUID loops without the use of external means. Additionally, no external circuitry is required to bias the SQUID loop.
An embodiment of a SQUID magnetometer according to the present invention can be fabricated by bi-epitaxial methods, although other deposition methods can also be utilized. For example, in the fabrication of a d-wave superconducting SQUID magnetometer according to the present invention, a seed layer may be deposited on a substrate and a first buffer layer may be deposited on the seed layer. In some embodiments, the seed layer may be MgO, the substrate SrTiO3 or Sapphire, and the first buffer layer CeO2. The first buffer layer and the seed layer may be etched, for example by Xe-ion milling although any appropriate etching method can be used, to form a boundary. The boundary separates a first area having the seed layer and the first buffer layer from a second area where the seed layer and the first buffer layer have been removed. A second buffer layer can be deposited on the second area and the first buffer layer. A superconducting material may then be deposited on the second buffer layer and etched, for example by Xe-ion milling although any appropriate etching method can be used, to form a loop and terminals. Resultingly, Josephson junctions are formed along the boundary. The boundary can be shaped so that both a 0-junction and a xcfx80/2 junction are achieved at the boundary. The second buffer layer may also be of CeO2.
The first buffer layer and the second buffer layer are deposited so that there is a lattice mismatch between the superconducting material deposited over the first area where the seed layer has been deposited and the superconducting material deposited over the second area, resulting in a grain boundary in the superconducting material at the boundary.